A boosted engine may exhibit higher combustion and exhaust temperatures than a naturally aspirated engine of similar output power. Such higher temperatures may cause increased nitrogen-oxide (NOX) emissions from the engine and may accelerate materials ageing, including turbocharger and exhaust-aftertreatment catalyst ageing. Exhaust-gas recirculation (EGR) is one approach for combating these effects. EGR works by diluting the intake air charge with exhaust gas, thereby reducing its oxygen content. When the resulting air-exhaust mixture is used in place of ordinary air to support combustion in the engine, lower combustion and exhaust temperatures result. EGR can also improve fuel economy in gasoline engines. At medium and high loads, fuel economy is improved due to knock mitigation, allowing for more efficient combustion phasing, reduced heat loss to the engine coolant, and lower exhaust temperatures—which in turn reduce the need for enrichment to cool the exhaust components. At low loads, EGR provides an additional benefit of reducing throttling losses.
In boosted engine systems equipped with a compressor mechanically coupled to an exhaust-driven turbine, exhaust gas may be recirculated through a high pressure (HP) EGR loop and/or a low-pressure (LP) EGR loop. In the HP EGR loop, the exhaust gas is taken from upstream of the turbine and is mixed with intake air downstream of the compressor. In an LP EGR loop, the exhaust gas is taken from downstream of the turbine and is mixed with intake air upstream of the compressor. Further, some engine systems provide so-called ‘internal EGR’, where combustion in one or more cylinders of the engine may be initiated when exhaust from a previous combustion is still present in the cylinders. The amount of internal EGR may be controlled using variable intake- and/or exhaust-valve timing.
HP and LP EGR strategies achieve optimum efficacy in different regions of the engine load-speed map. Moreover, each strategy presents its own control-system challenges. For example, HP EGR is most effective at low loads, where intake vacuum provides ample flow potential; at higher loads, the desired EGR flow rate may be unattainable due to reduced flow potential. Intrinsically dependent on turbocharger waste gate and throttle conditions, HP EGR may require a complex flow-control strategy. Further, HP EGR may suffer from poor EGR/air-charge mixing and may require a high rate of active cooling due the short length between the HP EGR take-off point and the intake runners of the engine.
In contrast to HP EGR, LP EGR provides adequate flow from mid to high engine loads in areas where HP EGR is flow limited, is more easily cooled, and can be controlled more independently of the throttle and waste gate. However, LP EGR may respond sluggishly to changing engine load, engine speed, or intake air flow. In gasoline engines especially, such unsatisfactory transient response may result in combustion instability during tip-out conditions, when fresh air is needed to sustain combustion but EGR-diluted air is present upstream of the throttle valve. Moreover, a significant lag in EGR availability can occur during tip-in conditions, as the amount of EGR accumulated in the intake manifold may not be sufficient to provide the desired combustion and/or emissions-control performance.
Turbocharged engine systems using more than one EGR mode have been described. For example, World Intellectual Property Organization Patent Application Publication Number 2007/136142 describes a system wherein a ratio of internal and external LP EGR is adjusted depending on engine operating conditions. However, this reference does not contemplate the full range of control options that are possible when fast-responding internal EGR is coordinated with slower-responding external LP EGR
Therefore, one embodiment provides a method for controlling combustion in a combustion chamber of a turbocharged engine. The method comprises admitting to the combustion chamber prior to ignition a first amount of exhaust from an intake manifold of the engine, the first amount changing at a first rate in response to a changing engine load. The method further comprises retaining in the combustion chamber prior to the ignition a second amount of exhaust from the combustion chamber, the second amount changing at a second rate, greater than the first rate, in response to the changing engine load. In this manner, external LP EGR can be used to supply a suitably dilute intake air charge under a range of steady-state conditions, while faster-responding internal EGR can be used to fill in during transient conditions.
It will be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description, which follows. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined by the claims that follow the detailed description. Further, the claimed subject matter is not limited to implementations that solve any disadvantages noted herein.